This invention relates to rotary power tools and, more particularly, to a new and improved control system and method to compensate for torque overshoot in such tools.
One area of use of this invention is in tightening threaded fasteners, although the principles of the present invention can be variously applied. In a typical power tool system to which the present invention relates, the fastening tool contains a torque transducer connected to a microprocessor-meter combination. In such a controlled tool, an analog torque signal is fed from the torque transducer in the tool to the microprocessor-meter in which a torque target has been set. When that target is reached, a control signal is sent from the meter to shut off the tool. In a fluid operated tool the shut off mechanism may be a solenoid-actuated valve in the tool. On an electric tool, shut off may be accomplished through an electronic switch. On a high torque-rate threaded joint, inertia in the decelerating high-speed elements in the tool can potentially drive the final tool torque well beyond the stall torque of the tool, and shut off or braking mechanisms cannot react quickly enough to prevent torque overshoot above the target torque set on the meter.
One way to correct the foregoing problem is to measure the amount of overshoot and adjust the set point on the meter downward so that the overshoot brings the final torque up to the target torque. Some presently available controls have built-in circuits to make this adjustment automatically, in effect learning the proper compensating set point after a few runs. The problem with this approach is that the torque rate of fastening jobs is seldom uniform, so that the amount of overshoot varies. A control which has learned to lower the set point from previous runs on high torque-rate jobs will yield a low final torque when the torque rate of the next job is suddenly low.
It would, therefore, be highly desirable to provide a system and method to compensate for torque overshoot which teaches the control the dynamic character of the power tool in use, in particular finding the curve, i.e., a mathematical function, which relates torque rate in the particular fastening job to overshoot in the tool. Then inspection or monitoring the torque rate of a job is effected while the tool is actually setting the job so that the torque set point can be lowered in precisely the correct amount to compensate for the overshoot related to that torque rate.